Many food products, such as meat and fowl, are transported in refrigerated condition by the application thereto of carbon dioxide snow. These products may be maintained either in completely frozen condition or at a temperature just above freezing, in both instances with the CO.sub.2 snow serving to compensate for heat leakage into the overall food product container.
Oftentimes such products are packed in containers having the shape of a rectangular parallelepiped which are transported along a conveyor in an open-top condition. The food products may have been previously chilled or frozen. When the product-carrying container reaches a station, its arrival is detected, and a proportionately sized hood is caused to descend and mate with the open-top container, or the container may enter an enclosure that can accommodate containers of various sizes. At this time, a charge of liquid CO.sub.2 is fed to one or more snow horns which include expansion nozzles through which the liquid CO.sub.2 expands to a mixture of CO.sub.2 snow and vapor. The snow is driven downward into the interstices of the product mass in the container, and the vapor exhausts through an exit, usually to a line leading to the exterior of the processing plant. Such equipment has been commercially available for over ten years and has established a place in the food processing industry.
Various attempts have been made to improve upon the basic equipment and provide more efficient usage of liquid CO.sub.2. For example, U.S. Pat. No. 3,932,155 discloses a snow-making system of this general type which takes liquid CO.sub.2 from the standard storage tank, that is at about 300 psig and 0.degree. F., and drops its pressure in an insulated chamber, termed a "subcooler," so as to create a reservoir of liquid CO.sub.2 at about 150 psig and about -30.degree. to -35.degree. F., which liquid CO.sub.2 is then subsequently expanded through a snow horn of special design to create "sofr" CO.sub.2 snow. A minor amount of precooling of the liquid CO.sub.2 before its drop in pressure is achieved by circulation through a hollow chamber surrounding the snow horn; however, this arrangement merely drops the temperature of the liquid CO.sub.2 one or two additional degrees F.
U.S. Pat. No. 3,660,985, issued May 9, 1972, shows somewhat related equipment wherein similar subcooling of a reservoir of liquid CO.sub.2 was carried out so as to lower its temperature; however, an arrangement was included for repressurizing the reservoir immediately prior to its use so the cold liquid being expanded would be at a higher pressure. U.S. Pat. No. 3,672,181 shows a tunnel-type CO.sub.2 cooler wherein the liquid CO.sub.2 has its temperature lowered on the way to expansion nozzles within the tunnel by flowing through a heat-exchanger located at the bottom of the tunnel onto which CO.sub.2 snow would gravitate if it fell below the conveyor belt. U.S. Pat. No. 3,754,407, issued Aug. 28, 1973, illustrates a system for establishing reservoirs of liquid CO.sub.2 at locations in the vicinity of snow-making equipment, which locations were distant from the standard CO.sub.2 storage tank. As a part of the system, a reservoir of lower pressure, colder liquid CO.sub.2 is established within an insulated heat-exchanger that is used to cool the high-pressure liquid CO.sub.2 flowing toward a holding tank that is located near the snow-making equipment.
It has been found that, in systems of this general type, the usage of which will be subject to the rate at which the food processing plant loads the food product-carrying containers onto the conveyors, there will be a considerable variance in the hourly rate at which liquid CO.sub.2 is expanded through the snow horns. Efficient use of this apparatus depends upon providing a metered charge of CO.sub.2 snow to each container so that both under-filling and over-filling are avoided and also depends somewhat upon efficient use of the liquid CO.sub.2. It is of course known that the quality of carbon dioxide changes with the change of its temperature, e.g., expansion of liquid CO.sub.2 at about -60.degree. F. may result in the creation of about 57 weight percent snow whereas expansion of liquid CO.sub.2 at about 0.degree. F. will create not more than about 47 weight percent snow. Ways for increasing the efficiency of snow-making apparatus of this type have been continuously sought after.